1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device having a memory device in which an electric charge discharging rate between two electrodes is different in accordance with the logic of stored information.
2. Description of the Related Art
Non-volatile memory devices that read out a difference in the discharge rate by applying a precharge voltage to a bit line are known.
As a representative example of the non-volatile semiconductor memory device to which such a reading method can be applied, there is an (flash) EEPROM.
On the other hand, in order to replace an FG-type (flash) EEPROM, as non-volatile memory devices that can rewrite data at a high speed, variable resistance-type memory devices have attracted attention.
As variable resistance-type memory devices, so-called ReRAMs are known in which a change in the resistance at the time of input/output of conductive ions to/from a conductive film disposed within a memory component is associated with a memory state (for example, see K. Aratani, etc. “A Novel Resistance Memory with High Scalability and Nanosecond Switching”, Technical Digest IEDM 2007, pp. 783-786).
In order to assure the reliability of rewriting characteristics, maintaining characteristics, and the like of the ReRAM and in order to apply the ReRAM further to a multiple-valued memory, as in a general flash memory or the like, a method in which a verify•read operation is performed at the time of a write operation or an erase operation is reviewed (for example, see JP-A-2009-26364, JP-A-2002-260377, and JP-A-2005-510005).
In the control of a current that is performed when a verify•read operation of a general flash memory is performed, a reading current (sense current) is approximately constant. Accordingly, by changing the electric potential of the gate of a memory transistor, a different threshold is verified. This operation method has the following merit. According to the operation method, the operating current is constant, and accordingly, the sense timing, the load of the sense node, and the like are hardly dependent upon a threshold to be verified.